Like addicts jonesing for their next fix, bees just can’t get enough of neonicotinoid insecticides, the very same substances that may be killing them.

That’s the word from two new studies published today in the journal Nature that add to the long line of evidence that the controversial chemicals are contributing to the worldwide decline of bees.

In the first study, scientists from Newcastle University in the U.K. gave both honeybees and bumblebees a choice of food. One was a simple sucrose solution. The other was a sucrose solution laced with very low levels of either imidacloprid, thiamethoxam, or clothianidin—the three most commonly used neonicotinoid insecticides. The goal was to test the common assumption that bees are able to detect and avoid the toxins.

It turned out that that was far from true.

“The bees actually chose to drink more of the solution that contained the neonicotinoids,” lead scientist Geraldine Wright said during a press conference on Tuesday. “We were in shock.”

Over the course of their experiments, researchers found that honeybees showed a 10 percent to 15 percent greater preference for the toxin-laced solutions, and bumblebees showed up to a 40 percent preference.

Why did the bees choose to consume the insecticide-laced food? It wasn’t a palate preference. The researchers found that the neurons in the bees’ mouths did not respond to neonics. In other words, their mouths could not detect the insecticides.

“What I think is happening is that neonicotinoids are essentially having a pharmacological effect on the neurons in the bees’ brains,” Wright said. “Like nicotine, they’re essentially amplifying the rewarding qualities of the sucrose solution. The bees think it’s more rewarding, so they go back to that tube to drink more of it.”

Wright added that the neonicotinoids must have fast-acting effects. “As soon as it gets into their blood, they are getting a ‘buzz,’ ” she said.

In the wild, a preference for neonics can have particularly nasty side effects: As the bees consume more of the insecticides, they’ll get sicker, losing motor function and foraging abilities—something illustrated by the second study.

In that study, researchers from Sweden’s Lund University looked at how plants grown from clothianidin-coated seeds affected several bee species in the wild.

The results varied by species but were striking overall.

For wild bees, population density dropped by half, from 40 bees per 500 square meters to just 20. A species called solitary bees had dramatically reduced nesting rates. Bumblebee colonies, meanwhile, effectively stopped growing and reproducing, something researchers measured by weighing colonies.

“There was almost no weight gain in the colonies in the insecticide-treated sites,” said lead researcher Maj Rundlöf.

Although previous studies have shown that neonics have a dangerous effect on honeybees, Rundlöf’s study did not encounter anything notable. This is problematic, she said, “because we use honeybees when we want to assess the environmental impacts of pesticides.”

Wright called the cumulative effect of neonicotinoids a “conundrum” for bees.

“They’re attracted to this stuff that is actually having a negative effect on their motor function and on their ability to collect food and forage,” she said. “It’s making them sick, they don’t want to eat as much, but they still think that it’s more rewarding.”

That’s a story that any addict can relate to, and one that may pose problems for bees for years to come.

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